Metal ions can both activate and repress eukaryotic gene expression. In yeast cells, Cu activates metallothionein gene expression by directly interacting with the DNA-binding domain of the ACE1 transcription factor. We have obtained evidence that the 11 critical cysteine residues of ACE1 are organized into two subclusters that interact with distinct regions of the upstream activation sequence. Cu also represses the activity of several genes involved in Cu uptake and utilization, including the cell surface reductase gene FRE1. We have identified and cloned a new regulatory factor, ACU1, that is involved in this repression pathway. ACU1 is weakly homologous to ACE1, and contains a cysteine-rich motif characteristic of Cu-binding proteins. Thus, each of the proteins that we have shown to be critical for Cu homeostasis in yeast appears to bear a common structural element, which we have dubbed the "Cu fist".